udp: Add split/into_split for UdpSocket

[leshow]

Motivation

With #2779 we don't need the split API anymore, however it still exists for UnixStream and TcpStream. Why not also for UdpSocket? It uses regular references or an Arc in the same way that the other types do.

Solution

Write a simple wrapper over regular references for shared split or Arc for owned split. @Darksonn seemed to be on board for this, thoughts?

[leshow]

This code currently fails on the visibility of RecvHalf/SendHalf. Where should these types be exported?

[Darksonn]

They should be exported in a new module tokio::net::udp.

[Darksonn]

There appears to be a merge conflict.

[leshow]

I gave it a rebase off the latest master, didn't see any conflicts but it should now have the latest changes.

[carllerche]

Split exists for TcpStream in order to get access to the AsyncRead / AsyncWrite trait implementations. UdpSocket does not have this requirement. When would one split a UdpSocket? I would rather avoid code if we can.

[leshow]

It's not necessary, but UnixStream doesn't implement AsyncRead/AsyncWrite and it can also be split. The PR came out of a discussion with @Darksonn on discord a little while ago, I think the desire was to have a similar interface for all the net types. If it's not wanted feel free to close.

[carllerche]

UnixStream does implement the traits, e.g. https://docs.rs/tokio/1.0.2/tokio/net/unix/struct.ReadHalf.html#impl-AsyncRead

[leshow]

My mistake, I should have double checked before commenting.

[Darksonn]

Closing in favor of #3448.

[ta3pks]

I dont know why this is closed. There is no way to split a udp socket so how one writes something using udp sockets? @carllerche what makes you think udp sockets must be unsplittable ? one very simple usage is to pass two ends to two different tasks so simultaneous read/writes are possible

[Darksonn]

This was closed because you can already use an Arc to achieve the same purpose. See here for more.